For the measurement of the angular rate of airplanes or missiles, electrically restrained gyros are used. The movement of the gyro about an input axis perpendicular to the spin axis of the gyro relative to a housing is picked-off by a pick-off, which provides a pick-off signal. This pick-off signal is applied to restraining electronics, which provide a restraining signal. The restraining signal is applied to a torquer. The torquer exerts a torque on the gyro about an axis perpendicular to the input axis and the spin axis in a way that the gyro is electrically restrained in a position of rest relative to the housing. The torque generated thereby, which in turn is represented by the current supplied to the torquer, ideally is proportional to the angular rate of the gyro and the housing about the input axis. In known electrically restrained gyros the restraining electronics is an analog amplifier network which provides a current directly supplied to the torquer with the desired time response. When the signal processing is digital, this current is converted by an analog to digital converter to a digital output signal.
Inertial sensor arrangements are known in which the angular rates about three mutually orthogonal axes are measured. This can be achieved by means of three electrically restrained gyros of the kind described hereinbefore. But also a gyro can be used which has two input axes which are mutually orthogonal and perpendicular to the spin axis. A pick-off and a torquer is provided on each of the input axes. The pick-off signal of the pick-off of one input axis is applied, through the restraining electronics, to the torquer which acts about the other input axis and vice versa. In this way the rotational movements about two coordinate axes are detected by one gyro, such that the intertial sensor arrangement requires only two gyros in all.
The prior art inertial sensor arrangements present some problems:
In gyros of the present type an extremely large dynamic range has to be covered. This dynamic range may be larger than 1:50,000. Therefore correspondingly extremely high demands are made on the accuracy, linearity and constancy with temperature variations. This is also true for the temperature- and long-time stability of the restraining electronics.
Gyro errors--except for the temperature compensation--in practice cannot be compensated for in the restraining electronics. Such error correction has to be made in a digital computer to which the gyro signals are supplied. This is particularly true for gyro errors which depend on other influencing variables. Such influencing variables are e.g. linear accelerations. Gyros furthermore show the so called "cross coupling behaviour". In the case of dynamically tuned gyros, a rotational movement about the first input axis causes a torque about the second input axis. Hereby, because of the gyro mechanics (nutation) and the restraining loop structure, a reaction results which causes a torque about the first input axis and thus is interpreted falsely as a rotational movement about the second input axis.
Further problems arise with the digitizing of the analog output signal. When the digitizing is carried out by a convential comparative analog-to-digital converter, a quantization error results which corresponds to the value of the least significant digit of the digitized signal. In most applications the angular rate signal provided by the gyro is integrated to obtain an angular position signal. A quantization error of the angular rate signal means, that the angular rates are not detected at all up to the value of the least significant digit of the digitized signal. The integration of the angular rate signals with respect to time results in a systematical error which corresponds to a gyro drift. These systematical errors are generally a multiple of what is admissible.
Therefore it is known to use an integrating voltage-to-frequency converter for analog-to-digital conversion. The current applied to the torquer is converted to a frequency proportional thereto. Then each output signal of the voltage-to-frequency converter corresponds to an angular increment. This method makes high demands on the voltage-to-frequency converter because of the extended dynamic range. As a frequency has no sign, the sign of the output signal has to be generated independently. This causes matching problems in the range of low angular rates. As low angular rates are output at correspondingly low frequency, only a coarse quantization results when the inertial information is detected at a high clock rate of e.g. several 100 cycles per second.
Thereby the frequency transfer behaviour of the restraining electronics is affected adversely such that the impulses for generating the output information provided by the voltage-to-frequency converter first have to be added up. This causes an additional delay between the occurance of an angular rate and the output of the digital output information. Such a delay involves a lag which adversely affects closed-loop control.